The present invention relates to a process for continuous production of metal foil clad and unclad laminates composed of several layers of thermosetting resin-impregnated sheet substrates.
The primary purpose of the present invention is to produce laminates for electrical insulation and printed circuit wiring uses.
Such laminates are generally required to have various characteristics including excellent electrical insulation, dielectric property, chemical resistance, surface smoothness, clad peel strength, and dimensional stability under various conditions. Further requirements comprise:
(i) to have excellent thermal stability to withstand solder temperatures as high as 260.degree. C., PA1 (ii) to generate neither unpleasant odors nor hazardous volatile matter when heated, PA1 (iii) to exhibit no large degree of warping adversely affecting printing and heating processes, PA1 (iv) to be easily subjected to punching processes, PA1 (v) to contain no bubbles impairing thermal conductivity and appearance, and PA1 (vi) to be as cheap as possible.
These laminates are usually provided with smooth surfaces, about 0.5 mm to 5 mm in thickness and about 1000 mm by 1000 mm in area.
According to conventional techniques in the art, unclad laminates are generally produced by processes wherein a fibrous substrate is impregnated with a varnish solution of a certain resin composition, dried to form a so-called "prepreg", and cut into a predetermined length, and several cut sheets, in turn, are stacked together and subjected batchwise to heat-pressing treatment. In such processes, however, the use of a solvent is essential for making up the varnish solution, and the prepreg has to be tack-free because of the restricted conditions of processing and workability. Consequently, these lead to the introduction of additional complicated plant, and to a considerable reduction in productivity.
In conventional processes, metal foil clad laminates also are produced through steps similar to the above, with the addition of a step wherein the stack is adhesively covered with metal foil, which has been preliminarily coated with an adhesive and heated to bring out the adhesive to its B stage. Although these clad laminates are used as printed circuit wiring boards and the like, they involve, in fact, problems concerning their productive efficiency and economy, because of the complicated batch processes and the high dependence on hand labor and skill.
Recently, in view of this, several continuous production methods for clad or unclad laminates have been proposed (for example, U.S. Pat. No. 3,236,714, U.S. Pat. No. 4,012,267, and Japanese Patent Publication No. SHO.53-88872).
All of these methods, however, have the following drawbacks preventing full realization of the economical and qualitative advantages inherent in a continuous production system, and are therefore only in limited use.
(a) In the case of using a varnish solution comprising a resin, the resin composition deposited in the substrate layer after drying, is, as a rule, nearly solid or an extremely viscous semifluid, and can hardly impart mirror-like surfaces to the layer. This surface roughness unavoidably allows voids or air bubbles to exist between the layers while the layers are stacked together. In order to remove completely such voids or air bubbles, the stack has to be subjected to heat as well as to a considerable degree of compression for prolonged periods during the curing step, which consequently necessitates the provision of highly complicated equipment. In addition, the provision of a drying oven and solvent recovery equipment diminishes to a large extent the advantages over the conventional batch system.
(b) When liquid thermosetting resin compounds are used instead, the above drying process is not always necessary.
On the other hand if the liquid resin generates gaseous or liquid volatile byproducts when heated, the necessity of applying prolonged pressure during the curing step still remains.
(c) Against the above difficult problem that pressure has to be applied to the continuously moving stack throughout the curing reaction, it might appear obvious to introduce a series of separate compressing devices, such as a serial combination of many pairs of heat-pressing rolls. Nevertheless, experiments have shown such a compromise is useless in providing laminates of high quality, because large periodical variations in pressure occur along the length of the moving stack, allowing partly entrapped inner bubbles to expand. Furthermore, it has been found that the periodic pressures applied to the resin composition in a hot fluid or semifluid uncured state imparted on undue local fluid mobility to the resin composition, resulting in intolerable undulations of the laminate surfaces. As a remedy, the continuous insertion of a highly rigid plate such as an iron plate between the stack and rolls has been tested to modify the adverse effects of localization and variation of pressure, but found to be less effective and at the expense of complicated equipment.